MC68HC908QL4 Datasheet, PDF (155/226 Pages) Motorola, Inc – Microcontrollers

Initialization/Application Information
At the time the ID is read, the user might also choose to read SLCBT and copy this value out to an
application variable. This data can then be used at a time appropriate to both the application software and
the LIN communications to adjust the trim of the internal oscillator. This operation must be handled very
carefully to avoid adjusting the base timing of the MCU at the wrong time, adversely affecting the
operation of the SLIC module or of the application itself. More information about this is contained in
14.9.16 Oscillator Trimming with SLIC.
If the user software determines that the ID read out of the SLCD corresponds to a command or request
message for which this node needs to receive or transmit data (respectively), it will then move on to
procedures described in 14.9.8 Handling Command Message Frames and 14.9.9 Handling Request LIN
Message Frames.
For clarification, in this document, “command” messages will refer to any message frame where the SLIC
module is receiving data bytes and “request” messages refer to message frames where the SLIC module
will be expected to transmit data bytes. This is a generic description and should not be confused with the
terminology in the LIN specification. The LIN use of the terms “command” and “request” have the same
basic meaning, but are limited in scope to specific identifier values of 0x3C and 0x3D. In the SLIC module
documentation, these terms have been used to describe these functional types of messages, regardless
of the specific identifier value used.
14.9.7.2 Possible Errors on Message Headers
Possible errors on message headers are:
• Identifier-Parity-Error
• Byte Framing Error
14.9.8 Handling Command Message Frames
Figure 14-16 shows how to handle command message frames, where the SLIC module is receiving data
from the master node.
Command message frames refer to LIN messages frames where the master node is “commanding” the
slave node to do something. The implication is that the slave will then be receiving data from the master
for this message frame. This can be a standard LIN message frame of 1–8 data bytes, a reserved LIN
system message (using 0x3C identifier), or an extended command message frame utilizing the reserved
0x3E user defined identifier or perhaps the 0x3F LIN reserved extended identifier. The SLIC module is
capable of handling message frames containing up to 64 bytes of data, while still automatically calculating
and/or verifying the checksum.
14.9.8.1 Standard Command Message Frames
After the application software has read the incoming identifier and determined that it is a valid identifier
which cannot be ignored using IMSG, it must determine if this message frame is a command message
frame or a request message frame. (i.e., should the application receive data from the master or send data
back to the master?)
The first case, shown in Figure 14-16 deals with command messages, where the SLIC will be receiving
data from the master node. If the received identifier corresponds to a standard LIN command frame (i.e.,
1–8 data bytes), the user must then write the number of bytes (determined by the system designer and
directly linked with this particular identifier) corresponding to the length of the message frame into
SLCDLC. The two most significant bits of this register are used for special control bits describing the
nature of this message frame.
MC68HC908QL4 Data Sheet, Rev. 7
Freescale Semiconductor
155